Gas turbine engine airfoils, such as turbine blades and turbine vanes, can be fabricated by investment casting. For instance, in investment casting, a ceramic or refractory metal core is arranged in a mold and coated with a wax material, which provides a desired shape. The wax material is then coated with ceramic slurry that is hardened into a shell. The wax is melted out of the shell and molten metal is then poured into the remaining cavity. The metal solidifies to form the airfoil. The core is then removed, leaving internal passages within the airfoil. Typically, the passages are used for cooling the airfoil.